Since the invention of ductile cast iron by Morrogh, et al., as disclosed in U.S. Pat. No. 2,841,488, many studies have been performed on the effects nickel has on ductile cast iron. Most of these studies use amounts of nickel in a range of percent by weight, that causes the iron phase to change from ferrite to bainite or austenite. It was found that by using nickel as an alloying element in ductile cast iron, several properties are improved, including corrosion resistance. In the past, although nickel has been known to improve the corrosion resistance of many materials, it was not thought of as being effective in ductile iron containing a ferritic phase.
In general with respect to ductile cast iron, ferrite is the major phase in an as-cast condition. However sometimes a self anneal in the mold or a post annealing process is necessary to attain the ferritic phase. The composition of ductile cast iron is similar to gray cast iron with the main difference being in the graphite structure. Ductile iron requires a nodularizing agent, such as magnesium or cerium, to produce a spheroidal graphite structure instead of a flake type of structure formed in gray iron. Because the graphite structure is not continuous and is forming a configuration which produces the least amount of graphite surface area, the ductility is increased extensively in the material.
However, ductile cast iron corrodes, and it has been found that ductile cast iron corrodes, in a manner, where micro galvanic cells are formed between the ferrite matrix and the graphite nodule. Because this galvanic action is dissolving the ferrite matrix, the graphite nodule becomes disconnected from the ferrite. Whether or not the nodule is pulled away from the surface or recombines with ferrite around it, a pit is formed. Once the pit is formed, an autocatalytic system is created and a larger pit appears. This situation can occur in most environments and especially in underground systems. The autocatalytic system is formed, because of the formation of an oxygen cell between the surrounding surface and the base of the pit. At the base of the pit, an acid is formed because of the high concentration of hydrogen ions. If salts are contained in an environment, such as those used for salting roads or that which is found in the ocean, then during this pitting process, hydrochloric acid is formed at the base of the pit.